1. Field of the Invention
The present invention relates generally to ion concentration measuring electrodes and, more specifically, to a sheet type electrode for measuring pH, pNa and the like, and methods for making the same.
2. Description of the Prior Art
A conventional electrode for measuring ions, such as pH and pNa, has been generally called a glass electrode and is shown in FIG. 12. The glass electrode comprises a support tube "a" formed of electrically insulating glass; a semi-spherical ion-response glass membrane "b" responsive to ions, such as pH and pNa, which is formed by the balloon method and joined to a pointed end of the support tube "a"; an internal electrode "c"; and an internal solution "d" enclosed in the support tube "a".
However, the above described ion-measuring electrode (glass electrode) having the conventional construction exhibits disadvantages. For example, since its ion-response glass membrane "b" must be formed by the balloon method, an adjustment of fire and blowing for adjusting a film thickness during its processing, or the prevention of microcracks which can be generated when the ion-response glass membrane "b" is joined to the support tube "a", require a considerable amount of skill in the art. Also, since it is difficult to mass-produce, not only is the cost of production remarkably high but also its construction cannot be other than large-sized, and its operation, maintenance and the like are also unsatisfactory.
As to the glass electrode having the conventional construction as shown in the above described FIG. 12, the semi-spherical ion-response glass membrane "b" having a film thickness (0.1 to 0.3 mm) could, for example, be realized by the balloon method even though it requires a considerable amount of skill in the art. In the event that the present invention of a sheet type glass electrode is sought to be produced, an ion-response super-thin glass membrane, such as pH-response glass membrane, is indispensable. The membrane must be flat and plate-like, and requires a film thickness near the limit of glassification. Such a membrane cannot be produced at the present, and thus the pending present invention of a sheet type glass electrode has not been realized.
That is to say, a thin plate glass having a thickness of at least about 1 mm has been produced by the vertical pulling-up method. A plate glass having a thickness less than the above described thickness (using, for example, a preparat and the like) has been produced by additionally subjecting the vertically pulled plate glass to a polishing process. However, in that case, not only is the cost of production remarkably increased but also relatively large uneven portions and cracks remain on the glass surface, and the resulting thin plate glass cannot be used as a glass-response membrane in an ion-concentration measuring electrode. Because of the large, uneven portions and microcracks in the plate glass, smooth absorption and desorption of the sample solution on and from the glass surface cannot be achieved, whereby accurate measurements cannot be achieved.
In addition, a prior method of producing a super-thin, flat plate glass includes a part of a circumferential surface of a super-thin, semi-spherical glass formed by the balloon method which is cut off and then reformed as a flat plate glass by means of a hot plate press. A prior art preformation method has also been used in which a previously molded glass is heated up to temperatures higher than a softening temperature to extend the glass. In the case of the former glass-reforming method, disadvantages occur in that a sufficiently large super-thin, flat plate glass is difficult to obtain and microcracks are apt to be generated. In the case of the latter preformation method, disadvantages occur in that a remarkably large-scale apparatus is required for production and a strain is apt to remain in the finished super-thin, flat plate glass. Moreover, both the above described methods exhibit a temperature-viscosity curve having a relatively gentle slope and they are effective merely for a normal glass (so called sodium glass) having a composition of a relatively wide melting temperature range. In the case where the above described methods are applied to a special glass (so called lithium glass) exhibiting a temperature-viscosity curve having a relatively steep slope and having a composition in which the melting temperature range is narrow, crystallization (detransparency) of the glass occurs, whereby it is quite impossible to obtain the desired super-thin, flat plate glass.
In addition, to realize a sufficiently reliable sheet type glass electrode, there is a problem in that a high electrical insulator must be secured between the flat, plate-like, ion-response glass membrane and a support layer and a substrate, as well as realizing the above described flat plate-like ion-response glass membrane itself.
The above described problem is also present in a composite electrode in which a glass electrode is integrated with a reference electrode.
The present invention was achieved in view of the above described state of the prior art and it is an object of the present invention to overcome the above described problems, whereby the present invention is a remarkably compact sheet type electrode which is superior in reliability, operation and maintenance, and can be easily and inexpensively produced.